Technical Field of the Invention
The present invention relates to a spark plug used an internal combustion engine such as an engine on a vehicle, and, an ignition system having the spark plug.
Related Art
A spark plug is known as ignition means for igniting air-fuel mixture introduced in a combustion chamber of an internal combustion engine such as an engine on a vehicle. The spark plug has, for example, a cylindrical housing, an insulator held inside of the housing and projected further toward a base end side than the housing is, and, a center electrode held inside of the insulator and projected further toward a head end side than the insulator is (refer to a patent document JP-A-H09-180856). Furthermore, the spark plug has a ground electrode forming a spark discharge gap between the center electrode and itself, and, a terminal portion held inside of the insulator, electrically connected to the center electrode and projected further toward the base end side than the insulator. The spark plug makes a part of the insulator to be inserted and fitted in a plug cap.
In recent years, with heightening efficiency of the internal combustion engine, technologies such as high compression ratio, high boost pressure and lean burn are applied, and demanded voltage of the spark plug (“demanded voltage” is voltage which is applied between the center electrode and the ground electrode, and needed for discharges to occur) tends to increase. Then, if the voltage of the spark plug increases too much, electrical insulation performance in the plug cap cannot be ensured enough, which might cause electrical leak between the terminal portion and the housing.
As a method for solving such a problem, for example, a method where extending the insulator of the spark plug toward the base end side and lengthening the axial length of the insulator lengthens electrical insulation distance between the terminal portion and the housing has been conceived.
In the above-described method, lengthening the electrical insulation distance between the terminal portion and the housing of the spark plug can increase the electrical insulation performance in the plug cap. On the other hand, the axial length of the portion where the insulator and the plug cap fit together is extended by the length of the portion extended toward the base end side in the insulator 12. This increases insertion and separation forces in the plug cap, which decreases attachment workability of the spark plug.